# rehomogenizePolynomial -- rehomogenization of a polynomial reversing the dehomogenization of the slack matrix

## Synopsis

• Usage:
h = rehomogenizePolynomial X
h = rehomogenizePolynomial(X, Y, T, g)
• Inputs:
• X, , a symbolic slack matrix X
• Y, , dehomogenized slack matrix Y
• T, , a spanning forest of the bipartite graph associated to X
• g, , a polynomial
• Outputs:
• h, , the rehomogenization of g reversing the dehomogenization of the slack matrix

## Description

Given the symbolic slack matrix X of a polytope, we may set to 1 the variables corresponding of a spanning forest of the bipartite graph associated with X. It can be useful to rehomogenize a polynomial in the variables that are left, reversing the dehomogenization process.

This function produces the rehomogenization of a polynomial following the spanning forest backwards. It is useful in the computation of the rehomogenization of the generators of an ideal, for example of the slack ideal.

 i1 : R = QQ[x_1..x_12]; i2 : X = matrix {{0, x_1, 0, 0, x_2}, {x_3, 0, 0, 0, x_4}, {0, x_5, x_6, 0, 0}, {x_7, 0, x_8, 0, 0}, {0, x_9, 0, x_10, 0}, {x_11, 0, 0, x_12, 0}}; 6 5 o2 : Matrix R <--- R i3 : (Y, T) = setOnesForest X; i4 : remVars := flatten entries Y - set{0_(ring Y), 1_(ring Y)}; i5 : h = rehomogenizePolynomial(X, Y, T, remVars_0^2+remVars_0*remVars_1-1) 2 2 2 2 2 2 2 2 2 2 o5 = - x x x x x x + x x x x x x + x x x x x x x x 1 4 6 7 10 11 2 3 5 8 10 11 2 3 5 6 7 8 9 12 o5 : R